1. Field of the Invention
The present invention concerns a radio-frequency transmission device for a magnetic resonance system to generate magnetic resonance exposures of an examination region of an examination subject of the type having a first radio-frequency transmission antenna to emit radio-frequency signals, a radio-frequency amplifier to supply the first radio-frequency transmission antenna with radio-frequency signals with a predetermined radio-frequency transmission power, and a second radio-frequency transmission antenna which is fashioned to mark a flowing medium in the examination region and/or regions by emission of radio-frequency marking signals, such that the medium is identifiable in the generated magnetic resonance exposures of the examination region. Moreover, the invention concerns a radio-frequency transmission antenna arrangement usable in such a radio-frequency transmission device, as well as a magnetic resonance system with such a radio-frequency transmission device.
Moreover, the invention concerns a method to generate magnetic resonance exposures of an examination region of an examination subject in which, in a magnetic resonance system, radio-frequency signals are emitted in the examination region with a first radio-frequency transmission antenna, and emitted magnetic resonance signals are thereupon received from the region and image data of the examination region are generated based on these, wherein a medium flowing in the examination region and/or regions is marked (via excitation of nuclear spins of the medium by means of radio-frequency marking signals emitted by a second radio-frequency transmission antenna) such that the medium is identifiable in the examination region in the generated magnetic resonance exposures.
2. Description of the Prior Art
Magnetic resonance tomography is a widespread technique to acquire images of the inside of the body of a living examination subject. In order to acquire an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or a body part of the patient to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field (usually designated the B0 field) which is generated by a basic magnetic field of the magnetic resonance system. During the acquisition of the magnetic resonance images, rapidly switched gradient fields (generated by gradient coils) are superimposed on this basic magnetic field for spatial coding. Moreover, RF signals (for example a radio-frequency pulse or a radio-frequency pulse sequence) of a defined field strength are radiated into the examination subject with a radio-frequency antenna. By means of this RF field (usually designated the B1 field), the nuclear spins of the atoms in the examination subject are excited such that they are deflected from their equilibrium state (which is oriented parallel to the basic magnetic field) an amount known as an “excitation flip angle” and precess around the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by radio-frequency acquisition antennas. The acquisition antennas can be either the same antennas with which the radio-frequency pulses are also radiated or separate acquisition antennas. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals. Every pixel in the magnetic resonance image is thereby associated with a small body volume (known as a “voxel”), and every brightness or intensity value of the pixels is linked with the signal amplitude of the magnetic resonance signals acquired from this voxel.
A particularly groundbreaking development of classical magnetic resonance imaging involves techniques in which the performance of marked blood in the brain is acquired with the use of a magnetic resonance apparatus. The blood supply in any arbitrary region of the brain can be determined by a subtraction of two images, one with marked blood and one without marking. Brain activities therefore can be mapped, or variations of the blood flow in pathological cases (such as given strokes, for example) can be detected. The observation of the performance of blood or other marked bodily fluids can also be reasonable in other organs in order in particular to be able to more easily detect pathological cases.
The marking of the blood has conventionally been conducted by the use of exogenous contrast agent based on gadolinium or the like. In order to able to forego the administration of such contrast agent, a technique known as the “ASL technique” (ASL=Arterial Spin Labeling) was developed, which is in particular used in the examination of the brain. The arterial blood is thereby electromagnetically marked (or “labeled”) by special excitation of the nuclear spins of the blood (for example in the neck region) before it reaches the brain. An image is acquired after a certain time period in which the blood labeled in this manner has distributed in the brain.
As described above, for this purpose a first radio-frequency antenna is required with which the “normal” imaging radio-frequency signals required for the magnetic resonance acquisition are emitted into the examination region, for example the head region of the patient or test subject. For example, this radio-frequency transmission antenna can be a “whole-body antenna” permanently installed in the magnetic resonance scanner and enclosing the examination space. It can also be a local antenna (for example a head coil) which is placed on the patient like a helmet during the examination. In such examinations it is possible to use the whole-body coil to emit the pulses and the head coil only to receive the magnetic resonance signals. The head coil, however, can also be used to send the radio-frequency signals and to acquire the magnetic resonance signals. The first transmission antenna which serves to emit the imaging radio-frequency pulses is designated in the following as an “imaging transmission antenna” or “imaging transmission coil”.
An additional second radio-frequency transmission antenna (designated as a “marking antenna” or “marking coil” in the following) that emits the radio-frequency signals used for the marking can be used for application of the ASL technique. This marking antenna is typically directly arranged locally on the examination subject, for example as close as possible to a suitable artery of the patient. It is usually a relatively small radio-frequency transmission antenna. An example of such a use of an additional radio-frequency transmission antenna for the implementation of a continuous ASL measurement is found in the article by Talagala S. L., Ye F. Q., Ledden P. J. and Chesnick S., “Whole-brain 3D performance MRI at 3.0 T using CASL with a separate labeling coil” in Magn Reson Med 2004:52(1):131-140.
However, present commercial magnetic resonance systems have only one radio-frequency transmission channel with a radio-frequency signal modulator and a downstream radio-frequency amplifier that is used to feed the imaging transmission antenna. In such apparatuses the question arises as to how the separate labeling antenna should be fed. As also described in the aforementioned publication, the marking antenna has conventionally been fed with an additional radio-frequency amplifier. This is shown in FIG. 1. Here a first modulator 4 and a first radio-frequency amplifier 3 exist in parallel for the first (the imaging) radio-frequency transmission coil 1. A separate modulator 6 and a separate radio-frequency amplifier 5 are available for the second radio-frequency transmission antenna 2 or, respectively, the labeling antenna 2. Significant additional costs thereby arise.